I just acquired a gross of M3 bulbs. The box says these bulbs have a light output of 16,000 Lumen Seconds, Peak Time 17.5 milliseconds. For all practical purposes that's very close to No. 5 bulbs. Can I assume my Guide Number will be the same ? I typically use GN 220 for 100 ASA film with No. 5's outdoors at night and in big rooms.

I know I should do a test but I thought I'd ask to see if anyone has any firsthand experience with M3's.

BTW: It's hard for me to believe that these tiny bulbs can pack a punch as bright as No. 5's.

I think you will find the M3s to perhaps need 1/2 to one stop more exposure than the #5. The biggest reason is the M3 was meant to be used in a smaller reflector, for a more narrow, concentrated, coverage. If you use a 5" reflector with them you should come close. The M3 does indeed pack a real punch for a small bulb. Those tiny AG1 bulbs are almost as bright as a #5, too. They were meant to be used in a little 2 1/2 to 3" reflector. The larger the reflector, the more exposure necessary for the same bulb. Good luck._________________Glenn

M-3 bulbs are filled with Rhenium which accounts for the "punch" they have. Peaking at 17.5 Milliseconds, I have used mine at 125th of a second with my Canon F-1 35mm camera with focal plane shutter. Using my pet load of 100 ASA Plus-X, I got a guide number of 160. In 35mm, I have used up to 1/500th of a second with M3 bulbs, but used a guide number of 80, which seemed to work fine. The M3 is my choice for 35mm.

For my Speed Graphic cameras, I still like the Sylvania Press 25s. I just started using them in the 7 inch reflectors now that I aquired a screw in adapter that accepts Press 5 and Press 25s. I did some tests and like the 7" reflector better indoors, since the spread angle is now 120 degrees.

They did make an adapter that takes M3 bulbs in the 5" reflector for the Speed Graphics. Never used one for 4x5._________________I love the smell of flashbulbs in the morning

They made a bayonet to M base adapter, but it also had a deep 3" reflector attached to it. These were, well inexpensively made, and lots of third party companies (back when Made in Japan meant cheap) made them out of plastic.

But Graflex also made a Mil Spec. M base adapter for the 2nd version of the 5" Graflite, the one with sheet spring fingers to hold the bulb. The M base adapter is machined out of stainless I think, and pops in snugly but still allows you to use the ejector button. Hard to find as most people don't know what they are._________________"In order to invent, you need a good imagination and a lot of junk" Thomas Edison